In an optical fiber transmission system, especially such a long distance transmission system as an undersea optical fiber transmission system, a fault detection technique remotely monitoring a condition of an optical transmission line using a reflection or scattering light (referred as a supervisory light hereinafter) of a signal light or as special signal for supervision based on C-OTDR(Coherent optical Time Domain Reflectometry) is widely known.
Further, an optical fiber transmission system has a pair of optical transmission lines consisting of an up flow use and a down flow use as a basis and in a long distance optically amplifying repeater transmission system, a structure wherein an optically amplifying repeater has a loop back circuit for transmitting a reflection light or a scattering light from the up system to the down system and also from the down system to the up system is well known. Such a loop back circuit is designed for sending back an output light from an optical amplifier and a return light resulted from scattering or reflection in the up system to the down system and also sending back an output light of an optical amplifier and a scattering or reflection light in the down system to the up system.
With a loop back circuit of a conventional system, transmission characteristics of each part of a fiber transmission line can be monitored at a terminal of a transmission end. There is no problem using the conventional system in the connection of two points. However, in case of providing one or more branch stations, measurement or monitoring in accordance with C-OTDR cannot be performed between an optically branching unit connecting a branch station to a primary optical fiber transmission (specifically, an add/dropping circuit for add/dropping a specific wavelength assigned to the branch station) and an adjacent optically amplifying repeater (precisely, a loop back circuit in it). The reason is that a scattering light in the section is sometimes cut off by an add/dropping apparatus.